Dechlorination Performance of Chemical Looping Conversion Using Red Mud as an Oxygen Carrier

Chemical looping (CL) conversion is a fuel heat utilization technology with CO2 capture and pollution control capability. During the treatment of Cl-containing organic solid waste, a large amount of HCl is generated, along with highly toxic polychlorinated dibenzo-p-dioxins and polychlorinated dibenzo-p-furans, thereby resulting in serious equipment corrosion and environmental harm. Red mud (RM) produced by the alumina industry is a potential cheap oxygen carrier (OC) with in situ dechlorination capability. In this paper, three types of RM are selected to study the evolution and varieties of Na- and Ca-containing compounds in the process of OC preparation and dechlorination. The dechlorination mechanisms and performances of the RM OCs under various reaction conditions are discussed. The results verify that the Na- and Na/Ca-rich RM OCs exhibit good dechlorination performances at a high reaction temperature. The RM OC components with dechlorination activity include Na in the form of soluble alkali (e.g., NaOH), Na2Ti3O7, and Na6Al4Si4O17, along with Ca in the form of CaO, Ca2SiO4, and CaTiO3, whereas NaAlSiO4, Na1.15(Al1.15Si0.85O4), and Ca2Al2SiO7 are inert. The dechlorination performances of the RM OCs are inhibited by the presence of water vapor and high temperature. In the dechlorination process using RM OCs, HCl first reacts rapidly with active elements on the surface of the OC with high dechlorination efficiency. With the saturation of surface-active elements, the reaction enters the HCl diffusion control stage, when the appropriate particle size of the OC and the gas residence time are helpful for improving the dechlorination efficiency.